1. Field of the Invention
The invention relates generally to automatic vent valves and more particularly to an automatic vent valve wherein a limited venting of a fluid system occurs on both the discharge and suction strokes of a positive displacement pump.
2. Description of the Prior Art
As a part of the manufacturing process for the production of semiconductors, photoresist, which is a fluid, is applied to silicon wafers. Typically, positive displacement pumps are used to meter the photoresist. An example of one such pump is disclosed in U.S. Pat. No. 4,139,333 issued to Sipos. An improved pump and fluid system is the subject of a pending application Ser. No. 340,644, now U.S. Pat. No. 4,483,665 by the present applicant.
When the photoresist is pulled into the positive displacement pump, it is subjected to a slight vacuum which allows some constituents of the photoresist to enter a gaseous state. The gases tend not to dissolve and result in bubbles in the photoresist applied to the wafers. This causes streaks on the wafers and renders them defective. The problem with gas coming out of solution is aggrevated as the altitude of the processing location increases.
Conventional photoresist dispensing systems use a manually operated vent valve. The manual vent valve is used primarily for venting the system when starting the pump. The valve is opened with its outlet directed to the supply of photoresist fluid. When a steady stream of liquid is observed, the system has been vented and the vent valve is shut. The manual vent valve is of little usefulness in venting gases during pump operation.
Another situation in which venting is necessary is when the photoresist fluid supply is changed. Typically, a hose runs from the pump suction to an open bottle of photoresist fluid. This hose must be removed and placed in a new bottle when changing photoresist supply. This operation introduces air to the hose which must be vented from the system. Generally, this is done by operating the pump for ten to fifteen strokes with the manual vent valve open. This results in lost operation time for the photoresist dispensing system.
In some photoresist fluid dispensing systems, the discharge of the pump is directed through a filter element. Typically, the filter medium is compressed by the fluid pressure created during the discharge stroke of the positive displacement pump. During the suction stroke of the pump, the filter medium tends to expand causing a pressure build-up in the system. Such a pressure build-up could cause dripping of the photoresist and render a wafer defective. One means for relieving such pressure build-up caused by filter expansion is disclosed in application Ser. No. 340,644 mentioned above.